Plant diseases reduce both the quantity and quality of plant products. Annual crop losses caused by disease amount to about 15% of expected harvest even in industrialized countries. The causes of plant diseases are many and diverse.
Infectious agents of plant diseases include such different groups as viroids, viruses, mollicutes, rickettsias, bacteria, actinomycetes, fungi, algae, protozoans, nematodes, insects and mites. A large percentage of all plant diseases is caused by fungi. These diseases are often spread by insects.
The soil microbe Bacillus thuringiensis (Bt) is a gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These inclusions can be observed microscopically as distinctly shaped crystals. The proteins are generally toxic to certain insects and contain specific endotoxic activity. Numerous Bt toxin genes have been isolated, sequenced, and recombinant DNA-based Bt products have been produced and approved for use.
Since the 1950's, formulations of Bt have been used as biological insecticides to control agricultural insect pests and more recently, insect vectors of a variety of human and animal diseases. Preparations of spores and crystals of B. thuringiensis var. kurstaki have been used for many years as commercial insecticides for lepidopteran pests.
Aflatoxins are highly carcinogenic compounds that are metabolic products of the ubiquitous fungi, Aspergillus flavus and A. parasiticus. Aflatoxins can contaminate peanuts and other crops such as corn whenever this commodity is invaded by the fungi. Reduction of aflatoxin contamination is a priority need in the production of peanuts (Arachis hypogaea). Aspergillus-like fungi invade peanut tissue when hot, dry weather prevails, especially during the last 4-6 weeks of the growing season. Fungal invasion of the developing peanut seed is facilitated when the pods are damaged in any way, such as feeding damage caused by the soilborne larvae of the lesser cornstalk borer (LCB), Elasmopalpus lignosellus, (Lepidoptera: Pyralidae).
Peanut pods and seed develop underground which makes preharvest control of aflatoxin contamination extremely difficult. Fungi that produce aflatoxins survive well in soil and invade developing peanut pods throughout growing season. Because no chemicals are effective against aflatoxin contamination of peanuts and culture control tactics are impractical, alternatives such as biological control are needed.
Organisms exhibiting biological control activity and natural products are among the control alternatives being sought for aflatoxin control. Organisms with biological control activity have an advantage of colonizing and, presumably, increasing in population as control is manifested. Natural products and biological control do not have the same negative impacts on the environment that synthetic pesticides may have. In addition, as synthetic pesticides become more difficult and expensive to discover and register, natural products are becoming increasingly important as alternatives.
One class of "biological control" compounds which have been investigated are the chitinases. Chitinases have been implicated in helping to control both insects and fungi, and also to help degrade crustacean exoskeltons, as they collect after the meat has been harvested from shellfish such as shrimp and crab. As a result, numerous chitinase genes have been cloned and sequenced from bacteria, plants, and actinomycetes. Interestingly, many of these chitinase genes display little sequence homology with other chitinase genes.
Therefore, there is needed biocontrol strains which are effective against insects and fungi.